I have an OBD-II interface for my laptop, and it's really only good for a few basic sensors... VERY limited. To get things like MPG, you need software designed FOR your car, specifically. Like, I can get LS1edit for my car, but it's still $400, and the newer, better stuff is about $800.
The basic, free software only gives you what was mentioned earlier, and is not that helpful, honestly.
I miss my OBD in my 1990 Firebird, because all you had to do was make a serial cable for $5, and hook it right to the laptop. Download a ROM off the forums, and you got access to EVERYTHING.
OBD-II has made things harder, and more generic, in my opinion. Most shops have paid for the
advanced versions of their OBD-II scanners, so they get a little more information--but I still don't think they get as detailed as stuff like LS1edit, which lets you
change your engine perameters, like fuel-mappings, timing, fans--pretty much everything on the fly
